This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Background: Vaginal rings are a promising delivery vehicle for microbicides because they may improve adherence, are coitally independent, and eliminate the need for daily application. Ethylene-vinyl-acetate (EVA) rings have been shown to release MIV-150 better than silicone. In the current study, we formulated an EVA ring with 100mg of MIV-150 and compared protection against SHIV-RT when macaques were challenged either 24 hours (24h) or 2wks PRI. Methods: Depo-Provera treated rhesus macaques (n=18) had either EVA/MIV-150 (n=7 each) or EVA/control (n=2 each) rings inserted vaginally before being challenged vaginally either 24h or 2wks PRI with 103 TCID50 SHIV-RT (SIVmac239 with HIV-1 RT). Infection was determined from plasma virus RNA and SIV-specific antibody (Ab) responses. MIV-150 absorption was measured by radioimmunoassay. Statistical significance was determined with Fisher's exact test. Results: In the test groups, 1/7 animals challenged 24h PRI and 1/7 animals challenged 2wks PRI became infected (2/14 total, 14.3%). In the control groups, 2/2 animals challenged 24h PRI and 1/2 animals challenged 2wks PRI became infected (3/4 total, 75%). SIV RNA and SIV-specific Abs were detected in the plasma of all infected (but not uninfected) animals. These data indicate that EVA/MIV-150 rings provided significant protection (p0.044) against vaginal SHIV-RT infection. MIV-150 was undetectable in the plasma 30min-72h PRI, suggesting that significant levels of systemic drug absorption were not required for protection. Conclusions: Vaginal rings releasing an NNRTI, MIV-150, are capable of blocking SHIV infection in macaques and warrant further development for the prevention of HIV infection. Future studies will optimize the dose of MIV-150 and the timing of ring insertion relative to virus exposure.